The T Field, located offshore east Malaysia, is a mature oil field that began its development in the 1970s. Conceptual geological models generally illustrated this field as a retrogradational turbidite setting. It is commonly known that the turbidite channel has enormous geological complexity and, therefore, presents great challenges for successful horizontal-well placement. This paper describes the first job in southeast Asia in developing horizontal-well placement in a turbidite environment.

Introduction

As of 15 years ago, T Field’s remaining oil deposits were present in the relatively thin turbidite channel complex on the steep-sloping flank of the field. Vertical and slanted wells were drilled to produce the targeted reserves, but post-drilling findings always showed inconsistency in sand existence and distribution because of lateral sand discontinuity.

Because of the limited infill reservoirs, high reservoir dip angle, and less-prolific productivity, drilling a highly deviated oil producer along the reservoir dip and steering along its productive layer in the oil leg are found to be the optimal drilling approach. However, high well-placement risks were assessed owing to uncertainties in lateral sand distribution, variable formation dip, and fluid contacts.

Evaluating Infill/Redevelopment Potential

Some reservoirs located within the western area were believed to carry more re-serves; production data indicated that downdip oil remains untapped by existing producers. An average dipping angle of 13 to 20° and gas-cap-dominant drive suggested that remaining-oil potential could be unlocked only through the addition of the drainage point. Material-balance analysis was conducted, which further verified the earlier deduction by showing that the targeted reservoir contains more oil in place compared with existing book figures and has reserves at approximately twice the amount of previous estimates made from decline-curve analysis.

A reservoir model was subsequently constructed. Two types of reservoir models were built in this exercise: a simple grid homogeneous model and a heterogeneous model that was created by referring to a geological depositional concept and to reservoir distribution based on a geostatistical method. Reservoir-modeling and -simulation results are consistent with those obtained from material balance. As expected, the secondary-recovery method will result in an incremental recovery because of the pressure maintenance and better sweep efficiency. However, this recovery is not ideal; the reservoir pressure was halved because of historical production, and lateral discontinuity still exists. Infill drilling was recommended as the best way forward.

Well-Concept and -Execution Strategy

The feasibility of executing the drilling campaign during the early stage of the study was also considered. In past development campaigns, deviated wells were drilled to penetrate and produce from multistacked reservoirs. The replication of such practices may result in missing the reservoir target because of the high potential of reservoir discontinuity.